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1. Field of the Invention
This invention relates to a method of splicing optical fibers and to apparatus for use in the method. More particularly it is concerned with the splicing by fusion of fibers that differ considerably in melting point.
2. Description of the Related Art Including Information Disclosed Under 37C. F. R. 1.97 and 1.98
Optical fibers of identical or similar melting point are routinely spliced by aligning the fiber ends and bringing the ends together while heating both of them to melting point, usually by means of an electric arc, a flame or a laser (usually a CO2 laser), so as to obtain a fusion bond.
There is an increasing need for splices between fibers with widely different melting points, and these are currently being made with adhesives or mechanical splices, which may be susceptible to ageing and/or damage from environmental conditions, because attempts to use fusion splicing resulted in splices with a degree of distortion at the interface that made their optical properties unsatisfactory.
JP57-24906A and EP0404152A disclose methods in which heating is concentrated at a point displaced from the fiber ends, so that the fiber of higher melting point is heated to a higher temperature than the fiber of lower melting point, but so that both fibers will be heated to their respective melting points. If the fibers are in contact during heating, some degree of heating by conduction of the fiber of lower melting point may be assumed to occur, but this will not be substantial as both fibers are to be heated directly by the arc or other heat source; and there is a risk of serious distortion if the fiber of higher melting point first softens at the place where heating is most intense, not at its end.
The present invention provides a means by which satisfactory fusion splices may be made between fibers with melting points that differ even by many hundreds of degrees.
The method in accordance with the invention comprises aligning the fiber ends and bringing the ends together while heating at least one of them to its melting point so as to obtain a fusion bond and is distinguished by the fact that the fiber of lower melting point is heated to a substantial degree by thermal conduction from the fiber of higher melting point.
It is neither necessary nor desirable to heat to the melting point of the higher-melting fiber.
It is to be expected that a small degree of upsetting of the fiber end of lower melting point will occur, but this can be controlled at a level that is not detrimental to the optical performance of the splice.
The invention includes apparatus for use in the method described comprising clamps for holding the fiber ends, adjusting mechanisms for aligning them, a mechanism for bringing them together and a heat source for heating at least one of them to its melting point while doing so, so as to obtain a fusion bond, and distinguished by the heat source being arranged to heat one of the fibers predominately, so that the other will be heated to a substantial degree by thermal conduction from the first-mentioned fiber.
Preferably the fiber of lower melting point is heated as nearly as possible entirely by conduction from the fiber of higher melting point.
It is unlikely that heat from an arc or flame could be localized to the required extent and so it is very much preferred, and might be almost essential under some circumstances, for the heat source to be a laser. An additional advantage of using laser heating is that the fibers can be clamped closer to their ends than when using arc or flame heating, and so precise alignment of the fibers can be better maintained during and after fusion.
Conventional lasers and control systems can be used, but we prefer to use the high-stability control system that is described in our European patent application EP1136855A (which is incorporated herein in its entirety by reference).
Preferably no part of either fiber is heated to a higher temperature than the end face of the fiber of higher melting point, and since it is impossible or at least difficult to obtain a beam with a sharp cut-off at the edge, this will normally require the use of a screen to establish a steep gradient of radiation intensity from a maximum near the end face of the fiber of higher melting point to near zero at the end face of the fiber of lower melting point (in relation to the initial positions of the fiber ends). If the beam has a sharp intensity maximum, it is preferable that it is centered close to the interface between the two fibers, so that about half the beam (at least to the extent it would fall on the fibers) will be intercepted by the screen; if the intensity of the beam is more evenly distributed, it may be desirable for the center of the beam to be slightly offset in the direction of the fiber of higher melting point.
In either case, because diffraction at the edge of the screen may be appreciable, the edge is preferably offset so that it projects slightly (a few micrometers or a few tens of micrometers is sufficient) beyond the edge of the fiber of lower melting point and preferably the screen is only slightly spaced (say a few tens of micrometers) from the surface of the fiber.
Since the screen should be heated as little as possible, it is preferably made of (or surfaced with) a material that reflects the laser radiation and inclined or shaped so that radiation falling on the screen is not reflected back to its source. It may be possible to arrange the screen so that some of the laser radiation falling on it will be reflected onto the end of the fiber of higher melting point. Stainless steel is a preferred material, but alternatively other refractory metals or tough ceramic materials might be used.
Except as otherwise described, the clamping, aligning, sequencing and movement of the fiber end and the apparatus for effecting them may be substantially the same as in conventional fusion splicers, but we prefer (as more fully explained later) to clamp the stripped fiber ends firmly and therefore to move the clamps holding them when bringing the fiber ends together, rather than clamp only the unstripped fiber ends and slide the stripped ends through loosely-fitting guides.